Quadrupole ion traps according to Paul and Steinwedel (DE-PS 944 900) consist of ring and end cap electrodes between which an essentially quadrupolar storage field is generated by applying RF voltages to the ring and end caps. Ions with varying mass-to-charge ratios (m/q) can be stored at the same time in this field (for the sake of simplicity, only "masses" instead of "mass-to-charge ratios" are referred to in the following since, in ion traps, one is predominantly only concerned with singly charged ions).
Physically intrinsic resonance conditions of the storage field are preferably used for ion ejection. With a pure quadrupole field, resonance conditions of this kind are found at the edge of the stability zone in the a,q diagram. In addition, with certain nonlinear conditions, in particular, those which occur in the case of a superposition of multipole fields, resonance conditions occur inside the stability zone and can also be used for ion ejection.
FIG. 1 shows some known storage field resonance conditions for a pure quadrupole field and for superposed hexapole and octopole fields plotted on an a,q stability diagram. The storage field resonances, .beta..sub.z =1 (for pure quadrupole), .beta..sub.z =2/3 (for hexapole superposition), .beta..sub.z +.beta..sub.r =1 and .beta..sub.z =1/2 (both for octopole superposition), have been plotted. The following applies in the customary manner: EQU a=-8zU/(mr.sub.0.sup.2 .omega..sup.2),q=4zV/(mr.sub.0.sup.2 .omega..sup.2)
where:
z=Coordinate of the rotationally symmetric axis of the ion trap, PA0 U=Direct voltage with which the RF storage field is superposed, PA0 m=Mass of ions, PA0 r.sub.0 =Inside radius of the ring electrode, PA0 2/3=Angular frequency of the storage RF, and PA0 V=Amplitude (voltage) of the storage RF PA0 quadrupole stability limit: f.sub.4 =F/2 for .beta..sub.z =2/2 PA0 nonlinear hexapole resonance: f.sub.6 =F/3 for .beta..sub.z =2/3 PA0 nonlinear octopole resonance: f.sub.8 =F/4 for .beta..sub.z =2/4 Coherent pushing of tile secular oscillation for an ion type optimally should take place very shortly (approximately 10 to 100 microseconds) before the storage field resonance is reached so that the coherently oscillating ions of the ion cloud are not again disturbed by collisions with the remaining gas. Irrespective of the scanning parameters (both frequency and amplitude scanning of the storage RF are possible), a secular ion oscillation frequency slightly lower than F/2, F/3 or F/4 is present just before resonance is reached. The excitation frequency, which must be applied to the two end cap electrodes in order to push the oscillation, has to be in resonance with this somewhat lower frequency. It therefore also has to be slightly lower than F/2, F/3 or F/4.
The advantages of these superposed multipole fields are discussed in detail in the International Journal of Mass Spectroscopy Ion Processes, J. Franzen, v. 106, pp. 63-78 (1991) which article is hereby incorporated by reference.
For measurement of the spectra, the ions are brought to a resonance condition of this kind mass by mass by changing the amplitude of the quadrupole RF storage field. When ions of a particular mass reach the resonance condition, they absorb energy from the RF storage field, enlarge their oscillation amplitudes and leave the ion trap through small holes in one of the end caps. The ejected ions can then be measured outside the ion trap with an ion detector.
The secular oscillation frequency of the ions varies widely after their production or introduction into the trap. Consequently, in order to provide a well-resolved mass spectrum, it is necessary to first collect the oscillating ions confined in the ion trap near the center of the ion trap to enable the ions of successive masses to leave the ion trap in ejection cycles clearly separated from each other in terms of time. For this, the ion trap is preferably filled with a special damping gas having an optimal density enabling the ions to release energy by colliding with the remaining gas in the trap. When such a gas is introduced, the trapped ions "thermalize" after a few collisions and collect at the center of the quadrupole field due to the focusing effect of the quadrupole field, reducing their oscillation amplitudes at the same time. They form a small cloud, the diameter of which is only approximately 1/20 to 1/10 of the dimensions of the trap according to tests carried out with laser beams as described in Physical Review A, I. Siemers, R. Blatt, T. Sauter and W. Neuhauser, v. 38, p. 5121 (1988) and Journal of the Optical Society of America B, M. Schubert, I. Siemers and R. Blatt, v. 6, p. 2159 (1989). Thermalization takes place particularly quickly with medium-weight damping gas molecules such as air.
However, in order to absorb energy under resonance conditions physically built into the storage field, the ions cannot be in a state of calm at the center of the quadrupole field since the RF field strength vanishes there and the ions are not affected by the intrinsic resonance conditions in the storage field. Absorption of energy is only possible as the ions move outwards from the field center and energy absorption actually increases the further the ions are from the field center due to oscillations.
It is therefore beneficial to intentionally weakly excite the secular oscillation of the ions shortly before they are brought to the resonance condition. This excitation is produced by subjecting the ions to resonance with a weak RF excitation voltage connected via the two end caps to produce an effective field at the center of the ion trap. Only this initial coherent excitation of the ions of a selected mass enables them to absorb energy from the RF storage field in the further course of the scanning process on reaching the resonance condition. In so doing, they are exponentially accelerated in the direction of the end caps and thus ejected from the ion trap.
Methods are also already known for removing ions the ion trap in resonance solely by the effect of the excitation RF applied (for example, as described in Center for Applied Physics, G. Rettinghaus, v. 22, p. 32 (1967). However, the absorption of energy solely from the excitation RF field essentially leads to a linear rise in secular amplitude. This compares to an exponential increase produced by the storage field resonance conditions. Consequently, ion ejection in the case of storage field resonances is very much sharper due to the field resonance and can be carried out in fewer oscillation cycles.
A simple scanning method with mass-sequential ejection of ions utilizing the limit of the stable storage range (.beta..sub.z =1) in the a,q diagram, without application of an additional excitation frequency for exciting the secular oscillation, has already been know for some time and is described in U.S. Pat. No. 4,540,884. A considerable improvement in the resolution of this method was obtained by the introduction of "axial modulation", which is a coherent excitation of the secular oscillation shortly before reaching the stability limit as described in EP-AL 0 350 159. Utilization of the nonlinear resonance, .beta..sub.z +.beta..sub.r =1, produced by superposing a weak octopole field onto the quadrupole field, is similarly well-known for ejection of ions after initial pushing of the secular oscillation as described in EP-AL 0 383 961.
As far as ion ejection is concerned, the nonlinear multipole resonance conditions and the resonance on the stability margin differ only in so far as the multipole resonances each show sharply defined singularities, while the stability margin, .beta..sub.z =1, of the quadrupole field sharply separates two large areas, one stable and the other unstable. In both cases, however, the ions experience conditions trader which they are able to absorb oscillation energy from the storage field.
As already shown above, the oscillation energy absorbed is all the greater, the further the ions are (at the maximum of their oscillation amplitude) from the center of the field. This absorption produces an exponential rising of the oscillation amplitude of the ions at these points. If all ions of the cloud are coherently pushed under the same conditions, they will continue to absorb energy practically synchronously. If the diameter of the cloud of the ions of a mass does not greatly increase but the oscillation amplitude increases considerably, all the ions will leave the ion trap in just a flew oscillation cycles. This produces a good mass resolution, even with very fast scanning methods.
In addition to the stability margin resonance, .beta..sub.z =1, of the basic quadrupole field, the hexapole resonance, .beta..sub.z =2/3, and octopole resonance, .beta..sub.z =1/2, are of particular interest (as shown in FIG. 1) since, in these three cases, absorption of energy of the secular oscillations takes place only in the direction of the symmetric axis between the end caps (so-called z axis) of the quadrupole field. Therefore, the ions leave the quadrupole field in the direction of the end caps without absorbing energy at right angles to this direction (in the direction of the ring equator, the so-called r direction). Consequently, the ions have a particularly high probability of leaving the ion trap through the small holes in the center of the end cap.
Since the secular frequency (f) is associated with the storage frequency (F) by the equation f=.beta..sub.z *F/2, the following secular frequencies are associated with the three storage field resonances stated above:
Experimentally, this method functions very well according to expectations, provided the scanning process itself is relatively slow. Here, a "slow" scanning process is to be understood as one in which the ions of a mass are ejected in a time longer than approximately 30 to 50 cycles of the secular frequency. The method is, however, of particular interest for extremely fast scanning processes in which only approximately 8 to 16 cycles of the secular frequency are used per mass. Experimentally, the ions of a mass can be almost completely ejected in approximately 5 to 7 oscillations. Surprisingly, very fast scanning processes unexpectedly produce measuring results which fluctuate considerably. The mass spectra are not quantitatively reproducible. The quantitative fluctuations of the results are many times higher than those of statistical forecasts based on ion counts.
A more careful examination of the ejection processes by means of computer simulations provides the reason for the considerable signal fluctuations with fast scans. To avoid fluctuations, it is not only necessary to excite the secular frequency of the ions before they reach nonlinear resonance, but the excited secular oscillation must also have a favorable phase position for the nonlinear resonance which is the same for all masses at the maximum of the ejection cycle. With a slow scanning process, the phase positions are averaged. This is not the case with fast scanning processes. The phase position continuously changes during the scanning process, i.e. the phase "runs through".